Plotting board



April 22, 1947. P. G. EDWARDS PLOTTING BOARD Filed May 6, 1944 /Nl E/VTOR P G EDWARDS 8V ATTORNEY Patented Apr. 22, 1947 PLOTTIN G BOARD Paul G. Edwards, Veron Telephone Laboratories, Incorporated,

a corporation of New York York, N. Y.,

a, N. J., assignor to Bell New Application May 6, 1944, Serial No. 534,487

Claims.

measurements of the explosion wave at a pick-up point.

In one method of training the crews of surface ships in depth bombing technique the surface craft maneuvers into position with respect to a cooperating submarine and makes a practice attack by dropping a sub-caliber depth charge which is exploded by water pressure at the agreed depth at which the submarine is operating. These charges are designed to produce uniform explosive pressur pulses which are picked up by directional hydrophones mounted on the sub marine.

As explained in detail in my copending joint application 515,888, filed December 28,

cated, of relative magnitudes indicative of the angle of wave incidence within of absolute values indicative of the distance from the pick-up point to the point 'of the explosion of the standard charge.

The practical value of this information in the large extent on the charge to fall to the assumed depth.

The problem of computing the error involved the quadrant and in a particular practice attack is further complicated by the fact that it is impracticable to design the practice bomb so that it will drop only to the level of the submarine in the time required for an actual bomb to fall to the much greater assumed depth. As a result of this the actual target in practice attacks often is a point, some distance ahead of the submarine, where a submarine travelling at the assumed depth would be at the instant an actual bomb would reach that depth.

The object of this invention is to simplify the procedure in converting the readings of the practice attack meter into terms of range and lateral error with respect to the actual target so that this information will be available promptly for the guidance of the crew of the attacking ship. According to the invention the readings of the attack meter are set up on a plotting board by means of a slider designed to indicate directly the location of the point of the explosion with respect to the location of the pick-up point on the submarine at that time. From this point so located a pivoted member is set to the course of the attacking ship in proper relation to a line representing the course of the submarine and a scale sliding on this member is set to intersect the true target position on the submarines course. The range and lateral errors can then be read directly from scales on the pivoted member and sliding scale and, if desired, the overall accuracy also can be obtained merely by scaling the distance between the explosion point and the true target. In the drawing Fig. 1 shows a plotting board according to the invention and Fig. 2 is a diagram illustrating the theory of operation of the board. In Fig. 1 the square base member I has a fixed center pin 2 representing the pick-up point on the submarine at the time of the explosion, an azimuth scale 3 concentric with the pin and a problem adjustment distance scale 4 extending diametrically through the pin and representing the course of the submarine under attack. Extending at right angles to each other through the center pin there are also two other scales 5 and 6 disposed at degrees to the line representing the course of th submarine. The orientation of these scales corresponds to the maximum sensitivity axes of the pick-up units and they are calibrated in terms of the attack meter readings.

The slider 1 is slidably and rotatably mounted aeiaaos on the center pin 2 by means of an elongated slot 8 in the arm 9. At the intersection of the center line ill of this arm and the center line H of the cross arm 12 there is a pin l3 on which there is rotatably mounted a protractor or azimuth scale M. The scale I5 is pivotally mounted on the pin l3 and carries a sliding scale i6 mounted at right angles to itself and capable of being reversed in direction as required. For convenience in using the board the scale I and all other movable scales are preferably made of a suitable transparent plastic material.

For a proper understanding of the use of this board to determine the accuracy of a practice bombing attack it is necessary to consider in more detail the manner in which the attack meter readings vary with the distance and direction of the explosion from the pick-up point on the submarine.

The pressure gradient pick-up units are mounted with their axes of maximum sensitivity at right angles to each other and each of these axes is at 45 degrees to the longitudinal axis or course of the submarine. Due to the cosine characteristic of the units, the response of either unit to a charge fired on its axis will be inversely proportional to its distance from the pick-up and the response of the other unit will be substantially zero. For a charge which is not on either axis the square root of the sum or the squares or the two responses is inversely proportional to its distance from the pick-up point and the ratio of the responses is the tangent of the angle between a line through the explosion point and the axis of one of the units. While each pick-up unit gives the same numerical response for each of two charges exploding in corresponding positions in opposite quadrants, there is no ambiguity as to the location of a particular charge on this account since the polarity of the response or each unit depends upon the direction from which the wave approaches the unit.

In Fig. l of the drawing the scale 5 is oriented to correspond to the axis of the A pick-up unit and the scale 6 therefore corresponds to the axis or the B unit. As indicated in the drawing charges exploding in the forward quadrant will give +A, +B readings and charges in the rear quadrant give i.., -13 readings where charges in the right and left lateral quadrants give A, +33 and +A -B readings respectively.

The response of the attack meter to a charge as explained above may be expressed as follows:

D B cos (l) and D sin 6 (2) Where DA and DB are the responses or the pick-- up units, K1 is a constant depending on the nature of the charge, the sensitivity of the pick-up system and various other known factors, D is the distance from the pick-ups to the point of explosion and 0 is the angle between one pick-up axis and the direction line to the explosion point.

From (1) and (2) which is the law or the attack meter as explained above.

Consider now Fig. 2 in which the lines A and B are at right angles to correspond with the axes of the pick-up units, and the line 0 is perpendicular to the lines (1 and e joining lines A and B. From the geometry of the figures a d c 5'75? 1 -6 and fie-C (6) b a (i -Hi (d c) =d -i2rlc+c (7) Substituting from (6) 2 2 b2 2 2 be a +b %+2c -=c(% +2+ 8) Dividing through by a 13 l 1 1 2 1 1 1 2 2 l 2 (b ns) 0 (t me o C b a 9) From Equation 11 it follows that if a and b are inversely proportional to the attack meter readings DA and D3 respectively then the explosion is located as being at f, and c is the distance from the explosion to the pick-up point.

Referring again to Fig. i. it will be seen that when the slider 'i is set with the line H on the proper attack meter readings on the scales 5 and 6 the pin i3 represents the location of the explosion with respect to the pick-up point 2 on the sub-marine. To avoid possible confusion the use of plus and minus attack meter readin s of the same numerical values is avoided by providing the meter with a scale, such as from 0 to 300, in which is Zero response, 0 is maximum negative response and 360 is maximum positive response as indicated by the numerical values on the meter scales 5 and 6.

For each practice attack the actual target will be some point on the scale 4 which is determined in accordance with the speed of the submarine, the direction or its course and the difference in the times required for the practice charge and a real depth bomb to sink to the assumed depth. In the case shown for purposes or" illustration this point is at +50 yards or 50 yards ahead of the actual pick-up point on the submarine. To determine the error with respect to this point, the scale i5 is put in place with the pin 13 in a hole at the zero point and the scale is aligned with the course of the attacking ship. This operation is greatly facilitated by the use of the protractor M which is rotated until its reading on the line ID of the slider 9 is the same as the reading on the azimuth scale 3, in this case about 251, at which time the radial line through the zero on the protractor is parallel to the submarines course. The angle between the courses of the two ships is determined by known means outside the scope of this invention and the scale 15 is then readily located by rotating it to this angle on the protractor.

The slider IE is then moved along the scale 55 until it intersects the true target position on the scale 4, in this case +56 yards. The range and lateral errors are thendirectly from the scales l5 and I6 and if required the overall error is determined merely by sealing the straight line distance between the target point and the pin IS.

The use of the board is not limited to the ranges indicated by the numerical values shown for illustration on the scales 4, l5 and I6 since in practice the sensitivity of the response of the attack meter may be adjusted and one or more suitable additional calibrations may be provided for the scales as required, In the case shown the hole in the scale I 5 for the pin 13 is at the zero of the scale on the assumption that the practice charge is dropped from the attacking ship at the dropping point of an actual bomb. Actual depth charges are usually dropped from the stern of the ship whereas it is sometimes preferable to drop the practice charges over the side. Any such lateral or longitudinal difierences in the dropping points may be compensated automatically by providing one or more alternate holes such as [1 and I8 suitably located with respect to the zero point of the scale.

What is claimed is:

1. A plotting board having scales intersecting in quadrature to represent the axes of a pair of pressure gradient microphones and calibrated in terms of the microphone responses to explosion waves of known intensity originating at different positions with respect to the microphones, a slider having arms intersecting in quadrature at a point representing the origin of an explosion wave and means for mounting one arm of the slider on the board for rotary and sliding motion with respect to the intersection of the scales to locate on the board a point representing the origin of a wave corresponding to the settings of the other arm of the slider on the scales.

2. A plotting board according to claim 1 in which the board has an azimuth circle about the point of intersection of the scales and the slider has an azimuth scale member rotatably mounted at the intersection of its arms.

3. A plotting board according to claim 1 in 4. A plotting board according to claim 1 in combination with a scale pivotally mounted on the slider at the intersection of the arms and a scale member removably mounted on the pivoted scale and adapted for sliding motion at right angles to the pivoted scale.

5. A plotting board having scales intersecting in quadrature at a point representing the location of a moving pick-up device, an azimuth circle about the point, a line through the point at 45 degrees to the scales representing the course of the pick-up device, a slotted member mounted for sliding and rotary motion about the point, a cross arm on the member, an azimuth scale and a first linear scale pivotally mounted at the intersection of the cross arm and slotted member and a second linear scale slidably mounted on the first at right angles thereto.

PAUL G. EDWARDS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

